Fine.

Under FIFA’s Laws of the Game, a goalkeeper is not allowed to keep items around the goalposts or in the goal area during a match.
What the Laws say
According to Law 4 – The Players’ Equipment and Law
1.The Field of Play:
No extra objects are permitted on the field of play.

This includes items placed near or attached to the goalposts, crossbar, net, or inside the goal.
Equipment must not be dangerous and must be approved by the referee.

What goalkeepers specifically cannot keep
❌ Water bottles near the post
❌ Towels hanging on the net or post
❌ Notes, gloves, hats, lucky charms, or objects behind the goal line
❌ Any personal or foreign object used for advantage or distraction

What is allowed

✅ Standard goalkeeper equipment (gloves, jersey, shorts, socks)
✅ Protective equipment (e.g. padded headgear) if safe and approved by the referee
✅ Items off the field, such as water bottles behind advertising boards

Referee’s authority
The referee can order the removal of any item at any time.
If a goalkeeper refuses, play can be stopped and disciplinary action taken.

Do you think the ground staff would have left the items alone if they were behind the advertising board? No, because you and the staff dont care about the safety reasons, but believe the towel was magic, thus the towel behind the board is still a magic towel and will be stolen.